This invention relates to ultrasonic diagnostic imaging systems and, in particular, to ultrasonic diagnostic imaging systems which produce volume rendered three dimensional images from polar coordinate image data.
Ultrasonic diagnostic imaging has reached a stage of evolution where real time three dimensional (3D) imaging is approaching practical reality. In 3D imaging the ultrasonic transducer scans a volumetric region to acquire a 3D set of ultrasonic echo data which adequately samples the volume. The 3D data set may be acquired by means of a one dimensional array which scans an image plane while the array is moved to sweep the image plane through the volumetric region. The 3D data set may also be acquired by means of a two dimensional array which electronically steers beams in three dimensions through the volumetric region. The 3D echo data set is then processed to form an image of the three dimensional region which can be displayed on a display device.
Since the display devices currently in use in ultrasound are monitors and flat panel displays, consideration must be given to the method for presenting a display of 3D information on a two dimensional display medium. Several types of 3D images may be formed from a data set. One is a projection, orthographic, or perspective view formed by the process known as volume rendering. Volume rendering is a technique for composing a set of samples in 3D space into a single 2D image for visualization. This is done by projecting a series of parallel raylines through the 3D data set from the perspective of a viewer who is viewing the volumetric region in the direction of the raylines, and accumulating some function of the voxels along each rayline. Each time a partial or complete new 3D data set is acquired the data must be rendered to create the next real time image in the sequence. Volume rendering is described in U.S. Pat. Nos. 5,474,073 and 5,329,929, for instance. Another 3D display technique is called multi-planar reformatting (MPR), by which one or more 2D image planes through the volumetric region are displayed.
Traditionally volume rendering or multi-planar reformatting is performed on 3D data samples or xe2x80x9cvoxelsxe2x80x9d which are arranged in a rectilinear grid in the spatial domain. There are a number of 3D acquisition methodologies, such as apparatus which acquires a sequence of parallel planes of parallel (linear) scanlines, which will directly produce 3D echo data in a rectilinear format. However, there are other acquisition methodologies that rotate or angularly fan a transducer array to scan a 3D volume which do not produce rectilinearly distributed echo data. Such data generally has polar or spherical coordinates in which individual scanlines or planes have an angular relation to a reference axis. Electronically scanned 2D arrays which scan a conical or pyramidal volume will produce 3D data with the same polar characteristic. In order to apply a conventional volume rendering algorithm to the data, it must first be converted to rectilinear or Cartesian coordinates. This process, which may be described as 3D scan conversion, is very computation intensive. Furthermore, the volume rendering algorithm may only need to operate on a portion of the 3D data set, meaning that much of the 3D scan conversion was unnecessary. Accordingly it is desirable to be able to do volume rendering of a 3D data set without performing needless, time-consuming scan conversion, and preferably without doing scan conversion at all.
In accordance with the principles of the present invention, volume rendering is performed directly on polar 3D ultrasound image data without the need for prior conversion to Cartesian coordinates. This is accomplished by warping the projected raylines of the rendering process to account for the physical relationship of the data. For example, instead of using parallel linear raylines, curved raylines are employed with a curvature which is a function of the scan acquisition geometry. The invention lends itself well to 3D ultrasonic imaging systems where high speed or low cost are desired.